Police are carrying out speed checks on roads across the North York Moors to try to cut the growing number of sheep being killed by motorists.
More than 200 sheep, including 125 lambs, were killed on roads in the moors last year.
A spokeswoman for the national park said the deaths were costly for farmers and could push them to take their flocks off the moors.
Drivers have been asked to take extra care when driving on unfenced roads.
Rachel Pickering, the national park's moorland project officer, said lambs become more adventurous at this time of year and are increasingly likely to dart out in front of vehicles.
She added: "The suffering experienced by the animals is immense. Vehicles involved in collisions also suffer considerable and expensive damage."
A North Yorkshire Police spokesman said excessive speed was the greatest cause of sheep deaths.
He said officers would be carrying out speed checks in the area over the next few weeks and additional warning signs advising that sheep may be on the road have been put up.